1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member, a process cartridge having an electrophotographic photosensitive member, and an electrophotographic apparatus.
2. Description of the Related Art
An electrophotographic photosensitive member is required to have sensitivity, electrical characteristics, and optical characteristics in accordance with an electrophotographic process to be applied. In particular, an electrical or mechanical external force, caused by charging, development with toner, transfer to paper, cleaning, and the like, is directly applied to the surface of an electrophotographic photosensitive member to be repeatedly used; therefore, the electrophotographic photosensitive member is required to have durability with respect to them.
More specifically, the electrophotographic photosensitive member is required to have durability with respect to the abrasion, scratches or generation of abnormal sounds on the surface of the electrophotographic photosensitive member, caused by the friction with a charging member, a cleaning member, a transfer member, and other auxiliary members, and the degradation of the surface caused by ozone and the adhesion of a nitrogen oxide both generated during charging (primary charging) of the electrophotographic photosensitive member under high humidity.
Recently, an electrophotographic apparatus has been commercialized, in which a gap between the contact charging member applied with a D.C. voltage or a D.C. voltage superimposed with an A.C. voltage and the electrophotographic photosensitive member surface is discharged, whereby an electrophotographic photosensitive member is charged. According to such a contact charging system, compared with a conventional corona charging system, oxidizing gas such as ozone and nitrogen oxide is less generated. However, binding of molecular chains of molecules constituting the surface of an electrophotographic photosensitive member is cut by high discharge energy. Therefore, there arises a problem in that the surface is degraded more.
Furthermore, there is also a problem in that toner adheres to the surface of an electrophotographic photosensitive member caused by repetition of development and cleaning. Regarding this problem, there is a demand that a cleaning property of the surface of the electrophotographic photosensitive member is enhanced.
As means for solving the above-mentioned problems, JP 05-053358 A discloses that a surface layer using a curable (cross-linking) resin as a binder resin is provided. However, with such a configuration, although the mechanical strength of the surface of an electrophotographic photosensitive member is enhanced, the problem caused by the adhesion of a so-called charging product formed from ozone and nitrogen oxide generated during charging tends to become serious. Accordingly, there is a tendency that a phenomenon occurs, such as image blurring under high humidity, a decrease in a transfer efficiency of toner, and an increase in a friction coefficient between the surface of an electrophotographic photosensitive member and a member contacting it.
Furthermore, JP 06-083094 A discloses that resin particles are included in a surface layer using a thermoplastic resin as a binder resin. However, according to this configuration, although the problem involved in adhesion of a charging product, such as a decrease in transfer efficiency is solved, it is difficult to remarkably improve the mechanical strength of the surface of an electrophotographic photosensitive member. Furthermore, such resin particles may cause light to scatter in a photosensitive layer to decrease an image quality, depending upon the particle size and the dispersion state, and an aggregation of resin particles may become a starting point of scratches of a photosensitive layer.
Furthermore, it is also proposed that silicone oil, stearate, or the like is added to a surface layer to decrease the friction coefficient of the surface of an electrophotographic photosensitive member. The addition of such a compound influences the movement of charge in a photosensitive layer, which causes a change in an image density due to an increase in a remaining potential, image blurring due to a decrease in an electric resistance, and generation of a ghost image due to charge remaining in a photosensitive layer. Furthermore, such silicone oil has a high surface transition, and is localized only in the vicinity of the surface of a photosensitive layer. Therefore, when the surface portion is worn out due to abrasion, the effect of addition of a compound is reduced. Particularly, in the case where a compound such as silicone oil is added to a surface layer, the contactness between the surface layer and a layer below is decreased, which may cause the surface layer to peel off.
Furthermore, in order to suppress the surface transition of a lubricant such as silicone oil, there is a method for dispersing particles having poor solubility among compounds having a small friction coefficient, in a surface layer. Such particles may cause diffusion of an electrostatic latent image due to light scattering and may generate scratching having a staring point of an aggregation, unless the particles are dispersed in a photosensitive layer uniformly. The problem of dispersibility of lubricant particles can be minimized to some degree by adding a dispersant; however, the dispersant may, in turn, prevent the movement of charge in the photosensitive layer and behave like an ion conducting agent under high humidity. Therefore, the dispersant inhibits the characteristics of electrophotography, to decrease a resistance of a surface layer, increase a remaining potential, and generate a ghost image.
Thus, hitherto, it has been difficult to enhance the mechanical strength and electrical strength of the surface of an electrophotographic photosensitive member, to decrease the friction of the surface with various kinds of contact members, and to enhance a transfer efficiency, without causing the problems such as degradation of an image due to light scattering, generation of scratches due to an aggregation, a decrease in resistance of a surface layer, an increase in a remaining potential, generation of a ghost image, and a decrease in contactness of a surface layer.